Lucas was never supposed to rule. He was never supposed to be the Crown Prince. The Crown Prince was always his older brother who was the oldest in the family. If he died, the position would pass to his oldest sister, who would be Crown Princess. Lucas was never taught the appropriate social graces when it comes to being Crown Royalty.
So, for the longest time, he was just this wild carefree blunt troublemaker. However, when the war started, his older siblings (the crown prince and the two oldest princesses as well the third princess) died in the beginning of the war, Lucas got this position thrust upon him. He managed to learn more while being the Crown Prince, but he still lacks the appropriate social graces to day.